Such an interface unit may, for example, be used for laser surgical cutting operations on the human eye. The interface unit, which in the case of a human object to be irradiated may also be referred to as a patient interface, is used in this case to establish positionally fixed coupling between the patient's eye and the laser system which provides the laser radiation, usually between the eye and a focusing objective of laser system. Fixed coupling is necessary in order to maintain a constant distance between the focusing objective and the patient's eye, so that the cut can be made with the desired high precision in the eye's tissue portion to be processed, for example the cornea.
For cutting operations not only eye tissue but also other biological tissue, as well as dead material, it is known per se to use so-called laser-induced optical breakdown. By applying focused laser radiation, such breakdown is created in the focal region when there is a sufficient spatial and temporal energy density of the pulses, lying above the breakdown threshold. Optical breakdown causes substantially athermic local destruction of the tissue being treated. This effect is referred to as photodisruption. By sequencing such photodisruptions, virtually any desired three-dimensional cutting pattern can be produced in the tissue being processed. Typically, laser systems having laser radiation pulse durations in the femtosecond range are nowadays used for cutting operations. For these ultrashort pulse durations, the breakdown threshold is comparatively low, which is conducive to low radiation exposure of the tissue being treated. The size of the photodisruption is essentially limited to the extent of the radiation focus. The cutting precision therefore depends crucially on the spatial adjustment accuracy of the focus.
It is possible to reference the eye's front surface in relation to the coordinate system of the laser system by using an interface unit of the type considered here. However, manufacturing tolerances of the interface unit, which can be manifested as tolerances of the optical properties of the interface unit, cause corresponding deviations of the focal position in the tissue being treated. The cutting precision therefore depends crucially on the manufacturing precision of the interface unit. A high manufacturing accuracy of the interface unit is therefore extremely desirable.
The interface unit may for example have a sleeve-like spacer piece and an optical window arranged on one end, in which case the laser radiation travels along the sleeve axis through the inner region of the spacer piece, passes through the window and then emerges from the interface unit. The outer side of the window is used for the object to be treated to bear on, for example the eye to be treated. The laser radiation therefore enters the material to be treated directly from the window. Correspondingly, the outer side of the window forms a positioning surface for positioning the object to be irradiated. On the other end of the sleeve from the window, the interface unit is furthermore equipped with suitable positioning structures for axial positioning of the interface unit in relation to the laser system. The spacer piece may for example be configured in the shape of a cylindrical sleeve; solutions in which a conical sleeve shape is selected for the spacer piece are known in the prior art, the window being provided at the narrow end of the cone. In these solutions, the spacer piece may be referred to as a spacer cone; it is to be understood that a conical shape for the spacer piece is in no way compulsory in the scope of the invention.
One way of manufacturing an interface unit configured in the above manner, with a spacer piece and a window for the radiation to pass through, consists in making the spacer piece from a metallic material, for example aluminium, and, for the window, fitting a glass plate which satisfies the optical requirements into a frame in the spacer piece and adhesively bonding it therein. Such a manufacturing method, however, places stringent requirements on compliance with permissible tolerances, because the individual manufacturing tolerances of the spacer piece and the glass plate can add together, and the adhesive bonding process can furthermore be another source of inaccuracies.